For analysis of viscous substances it is desirable to have a homogenous, or close to homogenous, sample. This is to ensure that analysis of the substance is not affected by an inconsistent sample. To ensure consistency of a sample requires thorough mixing of the substance, which is usually a fluid.
There are a number of methods of mixing viscous fluids to ensure an adequately homogenous sample.
The most common method is the use of a motor driven single blade, or a plurality of blades, through the fluid. This can require a rather large machine which is not always suitable if the container in which the fluid is held is of a small size.
Careful control of the motor speed is also required, to ensure that the sample is not excessively agitated, as this may lead to foaming of the sample.
A stirrer which utilises a motor for its mixing drive can also be noisy. This is a disadvantage if the apparatus is intended for use in a laboratory situation.
Another disadvantage of a motorised stirrer is that they require lubrication on account of its various moving parts. This lubricating agent can contaminate the container or the fluid being analysed, a concern if precise data is required. This is also a concern if the fluid is to remain food grade quality.
Another method particularly favoured in science laboratories for mixing or stirring of fluids is the use of a magnetic stirrer. These are capsule like magnets which are placed into the container holding the fluid to be mixed. The container is then placed over an electromagnet which causes a rotating magnetic field, thus causing rotational movement of the stirrer and subsequent mixing of the fluid.
These magnetic stirrers are quieter and more efficient than motorised stirrers, but none the less some problems arise in their use for mixing.
Many items of science glassware are provided with inlets or outlets at their base or around the lower portion of the sidewalls of the container. These allow additional fluids or substances to flow into or out of the container as required. Traditional magnetic stirrers and stirring paddles can obstruct these orifices when in use.
Traditional magnetic stirrers are also only suitable for mixing fluids of a relatively low viscosity. Fluids of a high viscosity can be difficult to overcome for the magnetic field causing rotation of the stirrer, unless a particularly large magnet is used. A larger magnet also requires a larger electromagnet to cause the rotational magnetic field.
A variation on the use of magnetic forces for mixing and stirring fluids is the method and apparatus disclosed in New Zealand Patent No. 516057.
This invention uses a paddle configured in the lid of a container in which the fluid to be mixed is held. A solenoid causes a pulsing magnetic field, which in turn causes a reciprocating motion of the paddle and subsequent mixing of the fluid.
A disadvantage of the aforementioned patent is that it is also necessary, if a range of glassware of varying sizes is used in a laboratory, to have a stirring mechanism which is specific to each size container.
The use of this method of mixing requires the container to be closed when in use. This is not advantageous in some situations which require additional substances to be added to the fluid while it is being stirred.
The use of a closed top container can also make it difficult for an observer to determine visually if the fluid is sufficiently homogenized.
At present, there are no provisions in mixing apparatus to allow for accurate determination of some of the properties of mixed fluids. Usually a visual check is performed as the fluid is mixed.
Alternatively, at stages throughout the mixing process, mixing is halted and a measuring instrument is used to assess the mixed fluid, with mixing recommencing if measurements deem it necessary. This can be time consuming, particularly if the process needs to be repeated several times before an adequate homogenized sample is realised.
Alternatively, other instruments, such as temperature gauges and other fluid parameter measuring instruments can be used to assess the mixed fluid. However, these, such as vertical descent spheres to assess fluid density or viscosity, can often impede or block inlet or outlet ports into the container, and this can affect the quality of the assessment of the fluid.
All of these problems arise with regard to the specific task of trying to assess the somatic cell count (SCC) of milk.
A typical way to gain an indication of SCC is to conduct what is known as the Californian mastitis test (CMT). This involves mixing a reagent (usually detergent based) with milk. If the milk has a high number of somatic cells (indicating a mastitic condition), then the milk/reagent mixture turns into a thickened jellified fluid. The properties of this fluid result from the strands of DNA from the somatic cells lysed by the reagent, tangling or coagulating. While sufficient mixing can provide an effective flow characteristic which indicates somatic cell count, too much mixing can cause the strands to break, thereby effectively thinning the fluid.
Thus, the fluid properties are difficult to define. It is not thixotropic as the cutting of the strands through over mixing means that the fluid does not return to a gel upon standing. Further, because the fluid is a mixture of DNA strands and other fluid, the fluid is not fully homogenous and is difficult to assign a viscosity reading.
Therefore, it can be appreciated that a solution to the aforementioned problems of not being able to have controlled mixing or measuring is desired.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.